Some embodiments relate to wide-field imaging.
More specifically, some embodiments relate to a wide-field imaging system for the infrared spectral range including a vacuum chamber that is optically open for the passage of the field rays originating from the scene to be imaged, a cooled dark chamber placed inside the vacuum chamber and provided with a cold diaphragm, an infrared detector placed inside the cooled dark chamber and a device for optically conjugating the field rays with the detector.
A related art system is disclosed in U.S. Pat. No. 4,783,593A related.
In particular, this patent describes an infrared wide-field imaging system including a vacuum chamber that is opened optically by a window which allows the passage of the field rays originating from the scene to be imaged into the vacuum chamber. Furthermore, this system includes a cooled dark chamber placed inside the vacuum chamber and provided with a cold diaphragm, an infrared detector placed inside the cooled dark chamber and a device for optically conjugating the field rays with the detector.
As illustrated in FIG. 3 of the U.S. Pat. No. 4,783,593A, the optical conjugation device mentioned includes a hot lens 16 (which is the camera lens of this device) which is placed outside the vacuum chamber and bears most of the optical power of the imaging system. In particular, the hot lens 16 forms a real intermediate image of the field rays originating from the scene to be imaged on a plane outside the vacuum chamber. The optical conjugation device further comprises a telecentric system for transferring this intermediate image. In particular, this telecentric system comprises a first hot lens 17 outside the vacuum chamber which collimates the rays originating from the plane of the intermediate image, and a second lens 18, called cold lens, inside the cooled dark chamber which focuses the rays collimated by the first hot lens 17 on the detector. The cold lens 18 is placed at a distance from the hot lens 16 greater than the focal length of the hot lens 16 and, furthermore, the distance between the cold diaphragm and the infrared detector of the cooled dark chamber is substantially equal to twice the focal length of the cold lens 18.
The effect of the optical conjugation device mentioned above is to form an intermediate image between the hot lens 16 and the first hot lens 17 of the telecentric system, so as to make it possible to change the field angle of the infrared wide-field imaging system by simply changing the objective lens (the hot lens 16). In particular, it is known that an objective lens with a short focal distance provides a wide field angle. On the other hand, an objective lens with a long focal distance provides a small field angle. Thus, by replacing an objective lens that has a short focal distance with an objective lens that has a long focal distance, the field angle of the imaging system is changed from a wide field angle to a small field angle.
Furthermore, it should be noted that, in the infrared wide-field imaging system of the patent mentioned, any error of positioning of the cold lens 18 in the cooled dark chamber, and therefore an error of the focal length of the system, is corrected by an adjustment of the position of the hot lens 16 relative to the cold lens 18, without requiring intervention on the cold lens 18 inside the vacuum chamber, this vacuum chamber generally being sealed.
However, this related art system presents a number of drawbacks, notably a significant length associated with the number of lenses and with the length of the cooled dark chamber, a high cooling time linked to the significant length of the cooled dark chamber, and a significant sensitivity to the temperature variations of the hot lens 16.